In a mountainous region that endures 300 freeze/thaw cycles a year, maintaining roads is an inherent challenge. Through its ongoing search for cost-effective ways to extend pavement life, Colorado DOT (CDOT) may have discovered its ideal solution: thin whitetopping.
Although asphalt has always been part of the state’s pavement structure, the pliable material is susceptible to damage from the harsh climate, snow tires, and chains that road agencies use several months every year. “When you have repetitive maintenance issues,” says CDOT pavement design program manager Jay Goldbaum, “you need to compare the cost and frequency of repairs with a long-term solution that costs more up front but saves money in the long run.”
CDOT began considering thin whitetopping as an alternative to asphalt overlays in 1990; by 2004, it was added to state specifications. To date, CDOT has placed almost 900,000 square yards of thin whitetopping.
“We base our decision to use thin whitetopping on a 40-year life-cycle cost analysis,” says Goldbaum (see chart, page 18). The agency assumes concrete overlays will last 27 years before requiring significant rehabilitation, compared to an asphalt overlay’s 12-year life span. Half of the state’s thin whitetoppings have been in service for a decade or longer, and have an average annual maintenance cost of less than $400 per lane-mile.
Goldbaum has noticed an interesting economic trend. “In areas where we’ve done whitetopping, we’ve seen hot-mix asphalt prices stay constant or even come down.”
Castle Rock Construction Co. of Colorado, based south of Denver in Centennial, has performed thin whitetopping for 22 years. Because CDOT doesn’t require specific concrete mixes, the contractor has optimized its own mixes to meet the overlays’ unique performance requirements. Castle Rock uses four aggregates that are blended based on workability and coarseness factors to create a well-graded mix. This requires stepped up aggregate gradation testing, usually three or four times a day, so the plant can vary the blend to meet the optimum factors. “This has a big impact on how the concrete reacts to dumping and vibration at the paver,” says COO Ralph Bell, “and creates the potential for significant gains in the pavement smoothness.”
Start with a good foundation
CDOT’s design guidelines require a minimum asphalt thickness of 5 inches, after milling or grinding, for a resurfacing project to qualify for thin whitetopping. A hot-mix asphalt base is required to avoid cracking, especially in areas with heavy traffic.
To ensure a good bond, the existing asphalt base must be milled to create a rough texture, and excess debris removed with a sweeper before placing the concrete. “The milling process establishes the bottom of the pavement grade, and must be carefully considered as to cross slope and longitudinal smoothing,” explains Bell. This step has a direct effect on the cost of paving, in terms of potential concrete overruns, and the thickness and ride of the pavement.
Thin whitetopping also can be placed over existing concrete, as a bonded or unbonded overlay. CDOT typically applies unbonded overlays, using a bond breaker, such as a thin layer (1 inch or so) of asphalt or fabric between the concrete road base and the surface layer, to cover existing joints. Without this layer, joints and cracks will be reflected in the new surface.
A bonded overlay, where a thin layer of concrete is applied directly to the existing concrete pavement, is most suitable when the existing pavement requires minimal repair. If a road has problem areas that need attention, individual panels can be replaced to create a uniform surface before the overlay is placed. With bonded overlays, all new joints have to line up exactly with the existing longitudinal and transverse joints—which is no small feat.
Joints are critical
Thin whitetopping requires close lateral and transverse joint spacing; the rule of thumb is one and a half times the depth, in feet. For example, a 4-inch thin whitetopping surface is cut into 6x6-foot panels.
“There were some early experiments with larger joint patterns that resulted in cracking, but performance has been good with this technique,” says Bell. “The contractor simply has to be prepared for the shear effort of so much sawing, and sealing joints afterward.”
When properly placed, a 6x6-foot pattern splits the wheel path in half, so drivers don’t feel the ridge and vehicles don’t stress the joint. According to Bell, the longitudinal joint in the middle of the driving lanes can be used successfully as a construction joint on roadways where a little more room is needed for the traveling public.
CDOT reinforces longitudinal joints with #4 epoxy-coated tie bars across the width of the road. No welded wire fabric or fibers are needed, and no dowels are used in transverse joints. They seal all joints with backer rod and self-leveling silicone sealant to reduce infiltration of moisture and incompressibles (sand and small stones) into the subgrade. They have found silicon works best, because hot-applied sealants aren’t compatible with the backer rod.
Goldbaum notes it may not actually be necessary to seal longitudinal joints, because they experience less movement than transverse joints. “We’ve left some longitudinal joints unsealed and they seem to be performing well,” he says.
In urban areas, where CDOT has placed most of its thin whitetopping, the department has developed a couple of ways to overlay existing curbs and gutters. One process covers the curb face with a mountable (angled) profile while the other process requires removal of the curb face. By removing the curb face, the contractor can reduce the vertical profile by 4 inches.
Castle Rock performed the first Denver-area overlay of an attached curb and gutter more than 10 years ago. The contractor prefers a sloping face curb, or “driveover,” design (CDOT Type IIM) because it is 10 inches from the flowline to the back of the curb, allowing more room and concrete thickness at the curb face. Bell notes several advantages to this approach:
No special equipment is needed.
- It saves time and costs of removing and preparing curb and gutter subgrade, therefore it’s less expensive.
- Curb-and-gutter subgrade is never subjected to water runoff from the overlay surface, or associated problems.
- No holes are left behind, which is safer for traffic and pedestrians.
- It eliminates relandscaping behind the curb and gutter (irrigation systems repair, etc.).
CDOT has overcome some of the barriers that discourage adoption of thin whitetopping.
Noise control. Transverse tining is the most popular way to texture concrete overlays. Although the technique meets Federal Highway Administration safety guidelines, it often results in noise complaints. CDOT used longitudinal tining—a quieter option—for 15 years, and began testing alternatives in 2003, including tining with sinusoidal waves, burlap, and diamond grooving and grinding. The agency’s preferred method is now Astro Turf: rough enough to add skid resistance but low noise.
“We are happy the tining is going away,” says Bell. “Not only because it impacts ride and noise—we always hated ripping up a nice slab.”
Local materials. In areas where aggregates are highly expansive, thin whitetopping overlays may require pressure relief joints similar to those used on bridges. High temperatures can cause the concrete to expand too much. “We’ve seen some heat-related slab blowups after several seasons, when incompressible material gets into joints and doesn’t allow them to expand,” explains Bell.
Education. The American Concrete Pavement Association offers resources for techniques and design, including a Concrete Pavement Restoration class and reference binder, and online Bonded Concrete Overlay on Asphalt Thickness Designer (visit http://apps.acpa.org) that calculates the amount of concrete needed, based on specific project parameters.
Bell’s advice: Keep it simple. “We encourage contractors and DOTs designing and building thin whitetopping to imagine they’re doing an asphalt mill and fill. A lot of the parameters are the same and one can get carried away with design.”
For Castle Rock’s customers, thin whitetopping is an economical option with consistent quality. “Hard decisions get harder in tough times,” says Bell. “But how do you ever catch the funding cycle unless you get more for the money you have?”